CHARACTERIZATION OF NMR DESHIELDING IN PHOSPHOLE AND THE PHOSPHOLIDE ION

Ab initio chemical shielding calculations employing the gauge including atomic orbtial (GIAO) method have been carried out on phosphole, the phospholide ion, and related thiophene, pyrrole, and furan compounds, to provide an understanding of the NMR shieldings observed in the phosphole systems. Structure and energy considerations in phosphole constrained to be fully planar (as opposed to being nonplanar and pyramidal) show that the system is conjugated but that this conjugation is in itself not responsible for the observed deshielding effect. Rather, it is shown that removing the stabilizing proton from the PH bond in phosphole to form the phospholide ion allows the system to become essentially fully conjugated and creates a lone pair only weakly coupled to the ring, and that this in-plane lone pair is responsible for the large downfield (paramagnetic) shifts seen in the ion relative to the parent compound.